1. Field of the Invention
The present invention relates to a secondary air supply control apparatus for controlling a secondary air flow supplied to an exhaust system of an internal combustion engine for purifying exhaust gas discharged from the engine.
2. Description of the Prior Art
Lately, as a measure for the purification of exhaust gas discharged from internal combustion engine, a secondary air supply apparatus has become increasingly employed which is destined to supply secondary air to the exhaust system from an air supply source constituted by an air pump. In this connection, the control of the secondary air supply is performed in dependence on the pressure prevalent in the intake or suction pipe (manifold) of the engine which is negative with reference to the atmospheric pressure and is in a proportional relationship to the revolution number of the engine in such manner that the secondary air supply to the engine exhaust system is permitted only when the negative intake or suction pressure in the engine intake pipe has attained a predetermined level and, if otherwise, the secondary air supply to the exhaust system is inhibited and bypassed to the atmosphere or back to the air pump. In the hitherto known control valve apparatus for effecting the secondary air supply control outlined above, there arises a problem that the air pump constituting the secondary air supply source is subjected to an overload when the pressure in the exhaust system of the engine is increased during the secondary air supply operation, resulting in a shortened use life of the air pump. As an attempt to evade such difficulty, it is known to provide a relief valve in combination with the secondary air supply control valve thereby to release a portion of the secondary air flow to the bypass conduit leading to the atmosphere or to the air pump when the pressure in the exhaust system increases beyond a predetermined level during the secondary air supply operation. However, with the provision of such relief valve, it has been impossible to reduce to a satisfactory manner the overload imposed on the air pump, because the relief valve tends to restrict the air flow passing therethrough to the bypass conduit due to the inherent operation behavior ascribable to the structure of the relief valve itself, as will be elucidated hereinafter.